It's been 47 years since we made our first footprints on the moon — here's what you might not know about Earth's dusty companion

NASA
It's one of the most defining moments in American history: On July 21, 1969, Neil Armstrong became the first person ever to walk on the moon, taking what he called "one small step for a man, one giant leap for mankind."

Forty-seven years later, we're still learning new things about Earth's dusty companion. And some of the things that we do know about it are pretty mind-blowing.

An astronaut's footprint can last a million years on the surface of the moon.

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It may have been decades since we last set foot on the moon, but its surface is still marked with the historic footprints of the 12 astronauts who stomped across it. That's because the moon has no atmosphere. It exists in the vacuum of space where there are no gentle breezes to sweep up the dust and erase the footprints.

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There are such things as moonquakes.

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During the Apollo missions, astronauts placed seismometers at their landing sights around the moon. They found that the moon actually experiences moonquakes, similar to earthquakes on Earth.

There are at least four different types of these moonquakes:

1. Deep quakes hundreds of miles below the surface, likely caused by tides.

2. Shallow quakes about a couple of dozen miles below the surface.

3. Vibrations that occur when meteorites hit.

4. Quakes caused when the sun heats up the moon's icy crust.

Some of these moonquakes can register up to 5.5 on the Richter scale, which on Earth would be powerful enough to crack plaster. Unlike earthquakes, which last up to only a few minutes, moonquakes can go on and on for as long as 10.

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The moon is not shaped like a perfect sphere. In fact, it's more like an egg.

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The moon is kind of shaped like an egg. That's because the moon's center of mass is actually a few miles closer to Earth than its actual geometric center.

In addition to being off-center, the moon also has a sort of lumpy gravitational field because of pockets of concentrated mass under some of its basins. One possible explanation for this is that asteroids hitting the moon billions of years ago caused regions of the moon to melt. As these regions dried, they became more dense and concentrated.

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There's no "dark side of the moon."

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You might know that the moon is tidally locked with Earth, which means that the same side of the moon faces Earth all the time. This is because the moon's own rotation matches its orbital period around Earth.

But just because the far side of the moon is always turned away from us doesn't mean that it's dark all the time. Over the course of a month, the moon will receive varying amounts of daylight, which is why we see different phases from Earth.

The only time the far side of the moon is actually dark is during a full moon or a lunar eclipse. In fact, during a solar eclipse, the far side of the moon is fully lit.

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The moon is getting further away.

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Every year, the moon moves about one and a half inches away from Earth. That's because of something called a tidal bulge, which forms in Earth's oceans because of the moon's gravitational pull.

Earth's rapid spinning drives the bulge forward so that it's slightly ahead of the moon. In return, the bulge feeds a tiny bit of energy into the moon, slowly pushing it away.

Think of it as if you're on a children's roundabout in a playground. The faster you spin, the stronger you feel a force pushing you outwards.

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There are "moon trees" planted all over Earth.

Ueslei Marcelino

During the Apollo 14 mission in 1971, astronauts carried a few hundred seeds with them to take into orbit around the moon. When the astronauts returned to Earth, the seeds, from loblolly pine, sycamore, redwood, Douglas fir, and sweet-gum trees, were then planted all over the US.

Many of these "moon trees" are still thriving today.

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There are many different ideas about how the moon came to be, and we don't know which one is right.

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Many scientists think that the moon coalesced from material blasted into space when a Mars-sized object collided with a smaller, baby Earth.

But other scientists think that it might have been preformed, wandering aimlessly through the solar system until it was captured by Earth's gravitational pull.

Still others think that the moon might have come from Earth itself, separating from it in the early days of the solar system.

There are also plenty of other hypotheses floating around about how exactly Earth's dusty companion came along.

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There are still tons of things we don't know about the moon.

NASA

For more than 4.5 billion years, the moon has been tagging along on Earth's orbit around the sun. Yet there are still plenty of things we don't know about it.

For example, the moon is covered in a spectacularly fine dust that stuck to the spacesuits of moon-exploring astronauts in the 1960s and '70s. When the astronauts returned to their spacecraft, the dust became airborne, finding its way into their lungs until they erupted in fits of sneezing and coughing.

Scientists are still trying to figure out what the effects of this lunar dust are on the human lung.